Popper and the Risky Test. You've covered modus tollens but stopped short of the inevitable: Popper's demarcation criterion[1]. A good test is a risky prediction — one the product could plausibly fail. Low-risk tests (tests that pass because there's nothing interesting they could catch) are the testing equivalent of unfalsifiable theories: they look like science but aren't. This would give you a rigorous vocabulary for the intuition behind "my regression suite is green but I don't feel confident."

[1] https://plato.stanford.edu/entries/popper/